Enhanced identification system

ABSTRACT

A method and an identification system used for communicating between a reader and a plurality of transponders are disclosed. The reader has a transmitter for transmitting a signal and each transponder includes a receiver for receiving the reader signal and a transmitter for generating a transponder signal. When the reader recognises a transponder signal from one of the transponders it immediately issues a mute instruction, muting all other active transponders and passing control to the said transponder, without the need for a specifically timed acknowledgement to the said controlling transponder. The reader may issue a single disable/wakeup instruction which disables the controlling transponder returning control to the reader and reactivated all muted (but not disabled) transponders.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of Ser. No. 11/392,429, filed Mar.29, 2006, which is a continuation of U.S. application Ser. No.10/635,683, filed Aug. 7, 2003, which is a continuation of U.S.application Ser. No. 09/415,234, filed Oct. 12, 1999, which is acontinuation of PCT/GB98/01385, filed on May 14, 1998, which are herebyincorporated herein in their entirety by reference.

FIELD OF THE INVENTION

This invention relates to a method of identifying a plurality oftransponders each of which transmits data at intervals to a receiver.The invention also relates to an identification system comprising aplurality of transponders and a receiver, and to the transponders andreceivers themselves. The invention further relates to method andapparatus for improving the identification systems disclosed in EP494,114 A and EP 585,132 A.

BACKGROUND OF THE INVENTION

Identification systems are known in which a plurality of transmitters,typically transponders, are activated by a power signal (or an“interrogation signal”) and then transmit reply signals, usuallycontaining identification data, to a receiver, which typically formspart of the interrogator. The signals may be transmitted in many ways,including electromagnetic energy, e.g. radio frequency (RF), infra red(IR), and coherent light, and sound, e.g. ultrasound. For example, thetransmission may be achieved by actual emission of RF energy by thetransponders, or by the modulation of the reflectivity of an antenna ofthe transponder, resulting in varying amounts of RF energy in theinterrogation signal being reflected or back-scattered from thetransponder antenna.

GB 2,116,808 A discloses an identification system in which theindividual transponders are programmed to retransmit data in apseudo-random manner. Timing signals for the transponders in thisidentification system are derived from a crystal oscillator, therebymaking the transponders expensive to manufacture.

EP 467,036 A describes another identification system which uses apseudo-random delay between transponder data transmissions. In thisexample, a linear recursive sequence generator is seeded by thetransponder identification address to make the pseudo-random delay asrandom as possible.

EP 161799 A discloses an interrogator/transponder system in which aninterrogator broadcasts an interrogation signal to a plurality oftransponders present in the interrogation field. Each transpondertransmits a reply signal consisting of a uniquely coded identificationnumber. The interrogator then re-transmits the signal it has receivedand each transponder decodes the signal and checks the data against it'sown identification number. In the event that a particular transponderrecognizes it 5 own code, that transponder discontinues the reply signalor adjusts to receive further instructions (all others having shutdown). If interference occurs because two or more transponders aretransmitting at the same time, the interrogator waits until a validsignal is received.

EP 494112 A discloses another interrogator/transponder system in whichan interrogator broadcasts an interrogation signal to a plurality oftransponders present in the interrogation field. One example of theidentification system comprises an interrogator or reader whichtransmits interrogation signals at a power of approximately 15 W and ata frequency of approximately 915 MHZ to a number of passivetransponders. The transponders derive a power supply from energy in theinterrogation signal, and modulate a portion of the energy received fromthe interrogator with an identification code to generate a responsesignal, which is transmitted back to the interrogator.

EP 585,132 A discloses another interrogator/transponder system in whichtransponders are provided with local timing means which is dependent onthe power supply voltage derived from the interrogation signal, therebycausing the clock frequencies of different transponders to varyrelatively widely. The interrogator is adapted to detect successfulreception of a response signal from any transponder and to derive asynchronisation signal from the response signal. The interrogationsignal may then be modified synchronously with a particular transponder.

The transponder can use separate receiver and transmitter antennas, or asingle antenna can be utilised for both reception and transmission. If asingle antenna is used the response signal can be generated bymodulating the reflectivity of such an antenna; if separate receiver andtransmitter antennae are used then a modulator which redirects energyfrom the receiver antenna to the transmitter antenna is required.Alternatively, the transponder can be independently powered and maygenerate its own response signal.

The system described in the above mentioned patent application providesfor each transponder to wait for a random or pseudo-random period afterreceiving an interrogation signal from the interrogator, beforetransmitting its own response signal. Successful identification of anytransponder is indicated by a brief interruption or other modificationof the interrogation signal, following closely on the successfulreception of a response signal of any particular transponder. This actsas a turn-off signal to the relevant transponder. The random orpseudo-random delay in the generation of response signals, in responseto repeated interrogation signals, ensures that all transponders willeventually be identified by the interrogator.

In general, if the transmissions of two transponders overlap or clash,the transmissions are polluted and therefore lost, since the receivercannot distinguish the separate transmissions. Thus, the system mustprovide for each transponder to transmit repeatedly until its entiretransmission takes place in a “quiet” time and is successfully receivedby the interrogator.

Any transponder must obtain a quiet time which is as long as the entirelength of the data stream to be transmitted. As shown in FIG. 1, thereis considerable wasted time in systems which employ a back-off and retryalgorithm of this sort.

EP 689 151 A2 discloses another interrogator/transponder system in whichthe RFID tag transmit a request to transmit (RTT) signal and wait for anacknowledgement signal from the network controller before attempting totransmit data. The disadvantage with such a system is that the tag mustwait for, and decode, an appropriately timed permission beforeattempting to transmit data, thereby adding unnecessary complexity tothe tag and leading to considerable wasted time in the transmissioncycle. If the tags are to have local timing means (as described indetail in EP 585,132 A) the timing and the duration of theacknowledgment instruction must be derived from the local timing meansof the tag transmitting the RTT signal. Since the RTT signal mustnecessarily be very short in order to provide the advantages suggested,the network controller must be able to extract the timing from verylittle information. This adds unnecessary complexity to the networkcontroller.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an improved, yet simple,identification system with improved recognition of data signals. It isalso an object of this invention to remove wasted time, to improve thespeed of identifying a plurality of transponders without addingunnecessary complexity to the identification systems.

It is also an object of the present invention to provide anidentification system in which wasted time due to transmission pollutionis considerably reduced while still providing a reasonably fast tagtransmitting cycle.

According to a first aspect of the invention, there is provided anidentification system comprising a reader including a transmitter fortransmitting a signal and a plurality of transponders, each transponderincluding a receiver for receiving the reader signal and a transmitterfor generating a transponder signal, characterised in that onrecognising a transponder signal from a transponder the readerimmediately issues a mute instruction, muting all other activetransponders and passing control to the said transponder, without theneed for a specifically timed acknowledgement to the said controllingtransponder.

According to a second aspect of the invention, there is provided amethod of identifying a plurality of transponders comprisingtransmitting a reader signal, and each transponder receiving the readersignal, characterised in that on recognising a transponder signal from atransponder the reader immediately issues a mute instruction, muting allother active transponders and passing control to the said transponder,without the need for a specifically timed acknowledgement to the saidcontrolling transponder.

In a further aspect of the invention, there is provided a transpondercomprising receiver means for receiving a reader signal, transmissionmeans for transmitting a transponder signal containing data whichidentifies the transponder whereby in a set of transponders, two or moretransponders may transmit their transponder response signals in responseto receiving the reader signal, characterised in that the transponder isprovided with control means, whereby on recognising a mute instructionin the reader signal, all active transponders in the set but one aremuted and control is passed to said one transponder, without the needfor a specifically timed acknowledgement to the said controllingtransponder.

In a yet further aspect of the invention there is provided an integratedcircuit for use in a transponder, comprising receiver means forreceiving a reader signal, transmission means for transmitting atransponder signal containing data which identifies the transponderwhereby in a set of transponders, two or more transponders may transmittheir transponder response signals in response to receiving the readersignal, characterised in that there is provided control means, wherebyon recognising a mute instruction in the reader signal, all activetransponders in the set but one are muted and control is passed to saidone transponder, without the need for a specifically timedacknowledgement to the said controlling transponder.

In a further aspect of the invention there is provided a readercomprising transmitter means for transmitting an interrogation signal toat least one transponder, at a time when at least one other transpondermay transmit in response to the interrogation signal, and receiver meansfor receiving a response signal from a transponder, characterised inthat on recognising a transponder signal from the transponder the readerimmediately issues a mute instruction, muting all other activetransponders, and passing control to said transponder, without the needfor a specifically timed acknowledgement to the said controllingtransponder.

The mute instruction may take the form of a total or partialinterruption in the reader signal, or some other modulation of thereader signal. Alternatively, the mute instruction may be a separatesignal transmitted by the reader, for example at a frequency whichdiffers from the reader signal.

An acceptance instruction may be sent after the transponder signal hasbeen successfully received by the reader. The acceptance instruction maybe of a similar nature to the mute instruction, such as a total orpartial interruption or other modulation of the reader signal.Alternatively, the acceptance instruction may be transmitted at afrequency which differs from the reader signal; such a frequency mayalso differ from the frequency of the mute instruction. The acceptanceinstruction may also have a different duration to the mute instruction,or be formed by repeating the mute instruction within a predeterminedperiod, e.g. using single and double pulses.

The mute instruction may mute the remaining active transponders byhalting the random wait cycle of the transponders until either restartedor reset by another instruction. If the random wait cycles of theremaining active transponders are halted by the mute instruction, theacceptance instruction may also instruct the remaining activetransponders in the reader field to restart the existing random waitcycles. Alternatively, the acceptance instruction may cause theremaining active transponders to begin new random wait cycles.

The mute instruction may simply mute a transponder by inhibiting thesaid transponder from transmitting. Any transponder which reaches theend of its random wait cycle is inhibited from transmitting thetransponder signal. For example, the mute instruction may set a flag andwhen a transponder reaches the end of its random wait cycle it checks tosee whether the flag is set before transmitting. The transponder inhibitmay be reset by the acceptance signal or be reset after a predeterminedtime.

The acceptance instruction may also act as a disabling instruction,disabling the transponder which has just transmitted either permanently,for a predetermined period of time or until it is reset. Thus a singleacceptance instruction may be used to disable a transponder which hasbeen successfully identified, and to instruct the remaining muted tagsto continue with the existing, or begin new, random wait cycles.

The transponders may, instead of being reactivated by an acceptanceinstruction to the controlling transponder, remain muted for apredetermined period of time. The transponders random wait cycle mayinclude a delay equal to the length of a transponder signal; if adisabling instruction is used then the delay may also include the periodfor the reader to transmit the disabling instruction.

If transponders with local timing means (as described in detail in EP585,132 A mentioned above) are employed, the acceptance instruction, ifused, may be synchronised with the particular timing means of thecontrolling transponder.

The frequency and duration (i.e. the timing) of the interrogatorinstructions may be preset in the interrogator at manufacture orinstallation. The timing may be set in an optimisation phase, e.g. afterinstallation or the timing may be optimised in an initial interrogationand used in subsequent interrogations.

If the transponders within the interrogation field are programmed withunique codes, the acceptance instruction may be eliminated altogether,and the transponders may thereby provide for the continued surveillanceof the articles to which the transponders are attached. If all thetransponders are programmed with the same code and the disablinginstruction is used, the number of transponders within the interrogationfield may be counted. The transponders may be used as a “presence tags”,indicating the number of articles to which the transponders areattached, and the response code may therefore be very simple. Selectiveuse of the acceptance signal may provide more flexibility to anidentification system.

Transponders and interrogators, such as those described in EP 494,114 Aand EP 585,132A may be adapted to produce transponders and interrogatorsaccording to the invention.

The invention will now be described in further detail. Specificnon-limiting embodiments are hereinafter described with reference to theaccompanying drawings, in which:

FIG. 1 is a simplified representation of prior art transponder datatransmissions;

FIG. 2 is a simplified block diagram showing an interrogator and threetransponders according to a first embodiment of the invention;

FIG. 3 is a simplified representation of an interrogator and a number oftransponders;

FIG. 4 is a block diagram of a transponder according to the firstembodiment of the invention;

FIG. 5 shows a timing diagram of a transponder according to the firstembodiment of the invention;

FIG. 6 shows a flow diagram for a transponder according to the firstembodiment of the invention;

FIG. 7 shows a detailed diagram of a gap detector circuit according tothe first embodiment of the invention.

FIG. 8 shows the circuit for the interrogator of the first embodiment ofthe invention.

FIG. 9 shows a timing diagram of a transponder according to a secondembodiment of the invention;

FIG. 2 shows an example of the RFID system comprising a reader 10,including a transmitter 11 with a transmitting antenna 11 a and areceiver 12 with a receiving antenna 12 a. The transmitter (11, 11 a)transmits a powering signal (the reader signal) to a number of passivetransponders (tag 1, tag 2 and tag 3).

Each transponder includes a dipole antenna, the two poles of which areindicated by 4 and 5. The transponders within the reader field are ableto derive a power supply from the energy in the reader signal using thecapacitor C and the diode D. The code generator 6 and logic circuit 7generate a signal using Manchester coding, which is transmitted to thereader, by modulating a portion of the energy received from the readerusing the modulator 9 connected between the antenna poles 4 and 5. Thetransponders have local timing means (as described in detail in EP585,132 A mentioned above).

On receiving power each transponder executes a random wait cycle beforetransmitting a signal. If a signal is received the reader issues a muteinstruction. The mute instruction may consist of a short gap (a partialor complete interruption) in or other modification of the signal. Allother active transponders within the reader field are temporarily mutedby the broadcast of the mute instruction, which is recognised as givingcontrol to another transponder. The reader issues an acceptanceinstruction (disable/wakeup instruction) once the transponder signal hasbeen received free from noise or interference. Since the transpondershave local timing means (as described in detail in EP 585,132 Amentioned above) the timing and the duration of this instruction issynchronised with the local timing means. The random wait cycles ofthese transponders are reactivated by this disable/wakeup instruction.

FIG. 3 represents the reader signal and the replies from thetransponders. The reader signal 20 is powered up a time to, whereuponthe transponders within the reader field are powered and begin randomwait cycles. In the example shown in FIG. 3, tag 1 transmits a signal attime t The reader recognises a transponder signal and, by interruptingthe reader signal at time t2, makes mute instruction 21 which halts therandom wait cycles of tags 2 and 3. When tag 1 has completedtransmitting the signal 20 the reader issues an instruction 22 at timet3.

In the example shown in FIG. 3, tags 2 and 3 are temporarily muted bythe broadcast of the mute instruction 21 at time t3. Tags 2 and 3 areinstructed to resume the random wait cycle by the broadcast of thedisable/wakeup instruction 22, which also disables tag 1 until it isremoved from the field. In this figure, the read process is thensuccessfully completed for tag 3 followed by tag 2.

FIG. 4 shows, in outline, tag 67 which may be used with the reader ofFIG. 2 and FIG. 5 shows a timing diagram for the signal paths marked inFIG. 4. The tag 67 includes a dipole antenna, the poles of which areshown as 60 and 61. A code generator 62, when enabled by a logic circuit64 modulates a transistor Q1 with a code, using Manchester coding(signal 77). The timing for the code generator is derived from a localoscillator 66. Diodes D1 and D2 in combination with a capacitor C1supply the power for tag 67. The oscillator is disconnected from therandom wait time generator when either FF1 or FF2 is in the reset state(signals 70 and 72). FF1 is set only when the tag 67 is powered up andresets when the tag 67 is switched off after being read successfully.FF2 is in the reset state when the tag 67 is muted and in the set stateon power up and when the tag 67 is in its normal operating mode. Whenthe tag 67 initially receives the reader signal FF1 will be in the setstate. On power up the logic circuit 64 triggers the random wait timer63 to select a random value and begin a countdown.

The gap detector circuit 65 is able to detect the presence or absence ofthe reader signal by means of diode D3 and, if there is an interruptionor gap in the reader signal, the duration of the gap. The gap detectorcircuit is shown in more detail in FIG. 7.

A long gap (a disable/wakeup instruction) will set FF2 and a short gap(mute instruction) will reset FF2. Therefore a disable/wakeup gap fromthe reader is detected by the gap detector which sets FF2, which enablesthe random wait time generator to run. The random wait time generatorindicates (signal 75) the end of the countdown to the logic circuit 64which then enables (signal 76) the code generator 62 to modulatetransistor Q1 with the code. The logic circuit also inhibits the gapdetector circuit for the time tag 67 takes to transmit the signal. If,during the countdown a mute gap from the reader is received (passingcontrol to another transponder) the gap detector switches FF2 into thereset state, thereby disconnecting the oscillator (signal 73) andpausing the countdown. FF2 remains in the reset state until anotherpulse from the reader is received (the disable/wakeup gap which disablesthe controlling transponder). The random wait timer then continues thecountdown until either the transponder signal is transmitted or anothermute gap is received.

If the reader issues an appropriately timed disable/wakeup gap once thetransponder signal has been received by the reader free from noise orother interference. The gap detector circuit detects this gap andindicates the presence of this gap to logic circuit 64. Provided thatthis gap occurs at a predetermined time after the end of the transpondersignal, e.g. 5 clock pulse after the end of the code, logic circuit 64(signal 71) will reset FF1. FF1 (signal 72) disconnects the oscillatoruntil it is reset, in this case after removing the tag 67 from the fieldand allowing capacitor C1 to discharge sufficiently.

If it is not necessary for the tags to be switched off after successfuldetection of the transponder signal, flip-flop FF1 and switch SW1 may beomitted entirely from the tag 67.

FIG. 6 shows a flow diagram for the operation of the tag 67 illustratedin FIG. 4.

The gap detector circuit 65 is shown in more detail in FIG. 7. When agap appears in the interrogation (a mute gap), the output of D3 goeslow. The edge detector circuit 102 detects a falling edge on the outputof D3, which in turn resets FF2. The output of OR gate 100 then passespulses from the oscillator 66 to the counter 101. When the counterreaches a value sufficient to make output Q4 go high, FF2 is set. Whenthe output of D3 returns to high (the end of the mute gap) the counteris reset. If the gap is short, counter 101 is rest before Q4 would havegone high. FF2 gets reset at the beginning of the gap and remains inthis condition after the gap. If the gap is long, FF2 is still reset atthe beginning of the gap. When counter 101 has counted up sufficientlyfor Q4 to go high, FF2 is set and it remains in this set state after thegap.

FIG. 8 shows the circuit for the interrogator. The transmitter 110produces a reader rf continuous wave signal which is transmitted to thetags via the circulator 111 and the antenna 112. The tag signal isreceived by the antenna 112 and passed via the circulator 111, toseparate it from the transmitter signal, to the mixer 113, whichextracts the low frequency code signal, and to the low pass filter 114.The mixers 113 mixes the tag's signal with a portion of the transmittedreader signal extracted by splitter 118 thereby generating the basebandsignal which is fed to filter 114. The outputs of the filter is thenamplified 119 and full wave rectified 121. The resulting signal is thenamplified and passed, via a divide by two circuit 125 to themicroprocessor 126. The microprocessor may interrupt the reader signalwith either a short or a long gap, using either the short monostable 127or the long monostable 128, the AND gate 129 and switch 130.

Referring to FIG. 9 in a second embodiment of the invention theinterrogator is adapted to provide a double pulse wakeup or acceptanceinstruction and single pulse for the mute instruction. The transponderis adapted to distinguish between the signals. In particular, in thetransponder the gap detector details circuit 68 is adapted to sense forthe occurrence of 1 or 2 “short” pulses of the same duration and toprovide the Set 5 signal to FF2 on occurrence of 2 pulses and the ResetR signal to FF2 on occurrence of 1 pulse.

It will be immediately appreciated by those skilled in the art thefunctionality of the tags and reader may be achieved in a number ofdifferent ways. For example, the tags, instead of deriving power fromthe reader signal, may be powered by a small battery.

In a further embodiment the integrated circuit of the transponder hasread/write capabilities. Instructions sent from the reader to thetransponder may take the form of coded instructions included in the muteand/or acceptance instructions. For example, if the mute and/oracceptance instructions are in the form of interruptions in the readersignal, the coded instructions may be present in the gaps in the readersignal defined by those interruptions.

In the embodiments above the reader, on recognising the transpondersignal from a transponder, immediately issues a mute instruction, mutingall other active transponders, and passing control to the transponder.Preferably the mute instruction is transmitted as soon as the readerrecognises, or there is a high probability that the reader hasrecognised, a valid signal from the transponder. In one embodiment thefirst few pulses in the signal from the transponder may have a uniqueshape or characteristic thereby enabling the reader to distinguishquickly whether or not the signal received is from the transponder asopposed to being the product of spurious noise.

1. An identification system comprising a reader including a transmitterfor transmitting a signal and a plurality of transponders, eachtransponder including a receiver for receiving the reader signal and atransmitter for generating a transponder signal, characterised in thaton recognising a transponder signal from a transponder the readerimmediately issues a mute instruction, muting all other activetransponders and passing control to the said transponder, without theneed for a specifically timed acknowledgement to the said controllingtransponder.
 2. An identification system as claimed in claim 1, whereinthe mute instruction comprises a modulation of the reader signal.
 3. Anidentification system as claimed in claim 1, wherein the reader issuesan acceptance instruction after the controlling transponder signal hasbeen successfully received by the reader.
 4. An identification system asclaimed in claim 1, wherein the transponder includes a random wait timerwhich triggers the transmission of the transponder signal after a randomwait cycle has been completed.
 5. An identification system as claimed inclaim 1, wherein the reader issues a disabling instruction after thecontrolling transponder signal has been successfully received by thereader.
 6. A method of identifying a plurality of transponderscomprising transmitting a reader signal, and each transponder receivingthe reader signal, wherein on recognising a transponder signal from atransponder the reader immediately issues a mute instruction, muting allother active transponders and passing control to the said transponder,without the need for a specifically timed acknowledgement to the saidcontrolling transponder.
 7. A method as claimed in claim 6, wherein anacceptance instruction is issued by the reader after the controllingtransponder signal has been successfully received.
 8. A method asclaimed in claim 6, wherein the transmission of the transponder signalby each transponder is triggered after a random wait cycle has beencompleted.
 9. A method as claimed in claim 6, wherein if a muteinstruction is received by a transponder before it has beguntransmitting its transponder signal, the random wait cycle of saidtransponder is halted.
 10. A method as claimed in claim 6, wherein if amute instruction is received by a transponder before it has beguntransmitting a transponder signal, at the end of the random wait cyclethe transponder is inhibited from transmitting the transponder signal.11. A method as claimed in claim 6, wherein a disabling instruction isissued by the reader after the controlling transponder signal has beensuccessfully received by the reader.
 12. A method as claimed in claim 6,wherein the controlling transponder is permanently disabled by adisabling instruction.
 13. A method as claimed in claim 6, wherein whena transponder detects a mute instruction the random wait cycle ispaused.
 14. A transponder comprising receiver means for receiving areader signal, transmission means for transmitting a transponder signalcontaining data which identifies the transponder whereby in a set oftransponders, two or more transponders may transmit their transponderresponse signals in response to receiving the reader signal, wherein thetransponder is provided with control means, whereby on recognising amute instruction in the reader signal, all active transponders in theset but one are muted and control is passed to said one transponder,without the need for a specifically timed acknowledgement to the saidcontrolling transponder.
 15. A transponder as claimed in claim 14,wherein the mute instruction from the reader comprises a modulation ofthe reader signal and the transponder has detection means forrecognising the modulation.
 16. A transponder as claimed in claim 14,wherein the controlling transponder is permanently disabled by adisabling instruction from the reader after the controlling transpondersignal has been successfully received by the reader.
 17. A transponderas claimed in claim 14, wherein the controlling transponder is disabledfor a predetermined period of time after the disabling instruction isissued.
 18. A transponder as claimed in claim 14, wherein thecontrolling transponder is disabled by the disabling instruction untilit is reset.
 19. An integrated circuit for use in a transponder,comprising receiver means for receiving a reader signal, transmissionmeans for transmitting a transponder signal containing data whichidentifies the transponder whereby in a set of transponders, two or moretransponders may transmit their transponder response signals in responseto receiving the reader signal, characterised in that there is providedcontrol means, whereby on recognising a mute instruction in the readersignal, all active transponders in the set but one are muted and controlis passed to said one transponder, Without the need for a specificallytimed acknowledgement to the said controlling transponder.
 20. Anintegrated circuit as claimed in claim 19, wherein the mute instructionfrom the reader comprises a modulation of the reader signal and theintegrated circuit has detection means for recognising the modulation.21. An integrated circuit as claimed in claim 19, wherein thecontrolling transponder is permanently disabled by a disablinginstruction from the reader after the controlling transponder signal hasbeen successfully received by the reader.
 22. An integrated circuit asclaimed in claim 19, wherein the controlling transponder is disabled fora predetermined period of time after the disabling instruction isissued.
 23. An integrated circuit as claimed in claim 19, wherein thecontrolling transponder is disabled by the disabling instruction untilit is reset.
 24. A reader comprising transmitter means for transmittingan interrogation signal to at least one transponder at a time when atleast one other transponder may transmit in response to theinterrogation signal and receiver means for receiving a response signalfrom a transponder, characterised in that on recognising a transpondersignal from the transponder the reader immediately issues a muteinstruction, muting all other active transponders, and passing controlto said transponder, without the need for a specifically timedacknowledgement to the said controlling transponder.
 25. A reader asclaimed in claim 24, wherein the mute instruction comprises a modulationof the reader signal.
 26. A reader as claimed in claim 24, wherein thereader issues an acceptance instruction after the controllingtransponder signal has been successfully received by the reader.